Project Summary Multiple Sclerosis (MS) is an autoimmune disorder that is characterized by inflammation of the central nervous system (CNS) and localized destruction of the brain and spinal cord resulting in debilitating neurological symptoms. The disease processes underlying MS have been studied extensively in an animal model called experimental autoimmune encephalomyelitis (EAE), which greatly helped to develop several approved therapies for MS. In both MS and EAE, CD4+ helper T cells mistakenly recognize myelin protein expressed by brain cells as foreign and mount an autoimmune response against it. This inflammatory response is mediated mainly by two subsets of helper T cells, so-called Th1 and Th17 cells, which infiltrate the CNS and produce the proinflammatory cytokines IFN? and IL-17, respectively. Interfering with the development of naive CD4+ T cells into Th1 and Th17 cells by genetic deletion of the transcription factors T-bet and ROR?t, respectively, protects mice from EAE. Our lab showed that CD4+ T cells require the influx of calcium ions for their activation and ability to produce IFN? and IL-17. Calcium influx is mediated by CRAC (calcium release-activated calcium) channels that are located in the plasma membrane of cells and formed by ORAI1 proteins. ORAI1 is activated by two intracellular proteins, STIM1 and STIM2, and deletion or mutagenesis of either of these three proteins attenuates CRAC channel function and calcium influx. Using mice with genetic deletion of ORAI1, STIM1 and STIM2 as well as specific CRAC channel inhibitors, we found that Th1 and Th17 cells are more dependent on calcium influx than other T cells for their function and ability to cause autoimmune inflammation. Treatment of T cells of mice or humans with a specific CRAC channel inhibitor reduced the production of IL-17 and IFN? but did not affect T regulatory (Treg) cells. in a dose dependent manner. By genetically ablating either ORAI1, STIM1 or STIM2 in T cells, we could prevent or ameliorate the development of EAE and CNS inflammation in mice. Importantly, deletion of ORAI1 or STIM1 in T cells after EAE symptoms had already developed stopped the progression of disease. Similarly, treatment of mice in which EAE had developed with a CRAC channel inhibitor also significantly reduced disease severity without apparent adverse effects. These data suggest that calcium influx is an essential mediator of CNS inflammation in EAE and that its inhibition may be a new option for the treatment of MS. Drugs inhibiting CRAC channels have been developed, but have not been tested for their efficacy and safety in the treatment of MS and other T cell mediated autoimmune diseases. The goals of this application are twofold: (1) To understand the molecular mechanisms by which calcium influx via CRAC channels controls the development of CD4 T cells into pathogenic T cells, in particular Th1 and Th17 cells, that mediate CNS inflammation and EAE/MS. (2) To evaluate CRAC channels as drug targets for immunotherapy of EAE and eventually MS by studying their role in human T cells, in a mouse model of relapsing-remitting EAE and in immunity to infections as a potential complication of antiinflammatory therapy.